I am familiar with that game, and what an autominer is. First remember that they track that and his account will get terminated for using such a program.
in order to help, what was the exact program he downloaded, and where did he download it from. that can be usefull, because i am sure this is not the first case, and it must be documented somewhere on the internet.
There are many ways to take care of this problem, but you said you have limited knowledge, so lets start with what program he downloaded and go from there.
Unsolved 
